The present invention relates to a shadow mask type colour cathode ray tube and more particularly to a curved shape therein.
A shadow mask employed in a shadow mask type colour cathode ray or picture tube is an important element possessing a colour selection function, In more detail, a shadow mask constituted by an effective surface portion that has a substantially rectangualr frame and has formed therein a large number of apertures in a regular array is provided at a set distance from a curved panel inner surface that has a substantially rectangular frame and has applied thereon individual phosphors for emitting a number of colours. A plurality of electron beams from electron guns provided in the neck portion of the tube are focussed and accelerated and are subjected to a deflection action cause them to scan a substantially rectangular area and to pass through the shadow mask apertures to strike and cause emission of light by corresponding phosphors and thereby produce an image. In order to ensure so-called beam landing between the set of shadow mask apertures and the set of corresponding phosphors, it is necessary that they be in a specific relative positional relation, which must be kept constant during operation of the picture tube. More specially, the interval between the shadow mask and the fluorescent surface (referred to as the q value below) must always be within a set permissible range. However, from the principle of operation of a shadow mask type colour picture tube, only one third or less of the electron beams pass through the shadow mask and the remainder strike portions of the shadow mask where there are no apertures, are converted to thermal energy and heat and cause expansion (referred to below as doming) of the mask. Consequently, if the position of the shadow mask, which is generally made of metal having iron as its main component, changes to the extent that the q value is outside the permissible range because of heating and expansion, the result is deterioration of the colour purity because of misalignment of the beam landing positions. The magnitude of this mislanding caused by thermal expansion of a shadow mask varies considerably depending on the image pattern on the screen and the length of time this pattern continues.
Mislanding caused by heating effects extending from the shadow mask to the mask frame which supports the shadow mask, and which possesses a large heat capacity, requires a comparatively long time and an effective method of compensating this is to include bimetal in the spring support structure mounted on the mask frame, as disclosed in Japanese Patent Publication No. 44-3547. However, mislanding that is brought about in a comparatively short time, e.g., local mislanding due to local doming caused by very bright local displays, is a considerable problem.
In connection with mislanding that occurs in a short time, if use is made of a signal unit for generating rectangular window-shaped patterns and the magnitude of mislanding is measured for different shapes and positions of the window-shaped patterns, it is found that mislanding is comparatively small when there is a large-current beam pattern 5 over practically the entire surface of the screen 6 as shown in FIG. 6 and that the greatest mislanding occurs when there is a large-current beam raster pattern 5 that is comparatively long and narrow and is displayed slightly toward the centre from the left or right-hand edge of the screen periphery as shown in FIG. 7. This can be understood from the following reasons.
Firstly, since a TV receiver is designed so that the picture tube's average anode current will not exceed a set value, the current per unit area of the shadow mask is smaller with a large window-shaped pattern as in FIG. 6 than it is in the case of FIG. 7 and so the temperature rise is small.
Secondly, if a pattern is in the middle of the screen, it if difficult for mislanding to occur even if the shadow mask is thermally deformed, but the degree to which thermal deformation of the shadow mask appears as mislanding on the screen becomes greater as the pattern moves from the centre towards the left or right-hand edges. However, actual deformation near the left and right-hand edges of the screen is small, since the shadow mask is fixed to the mask frame in these locations. Thus, the greatest mislanding occurs in the case of window-shaped patterns in a position like that shown in FIG. 7.
FIG. 8 is a drawing for the purpose of explaining the form mislanding takes in the case of a pattern such as shown in FIG. 7. A shadow mask 136 is held in a facing relation to the inner surface wall of a panel 124 by a mask frame 134 making use of stud pins 125 and spring support structures 135.
During operation at low luminance, i.e., when the electron current density is small, the shadow mask 136 is in position a1 and an electron beam 142 at position c1 passes through an aperture 137 and lands correctly on a corresponding phosphor dot 130. On a change from this state to display of a pattern with high local luminance such as shown in FIG. 7, local heating and expansion of the shadow mask 136 occurs, so resulting indisplacement of the shadow mask to position a2 and displacement of the aperture 137 from position b1 to b2, in consequence of which the electron beam 14 that passes through the aperture 137 shifts from position c1 to c2 and there is no longer accurate landing on the set phosphor dot.
There is a procedure employed for preventing this short time thermal deformation of the shadow mask, which is to make the portions where the shadow mask is fixed to the mask frame as flexible as possible so that instead of there being doming deformation indicated by the dashed line 136a in FIG. 9(a) the shadow mask 136 as a whole moves parallel to the tube axis as indicated by the dashed line 136b in FIG. 9(b). However, although such a measure is effective against displacement caused by thermal expansion of the whole surface of the mask as in FIG. 9(a) or (b), it is of practically no effect against local displacement such as occurs in the case shown in FIG. 7. This trend becomes more marked as tubes are larger and have larger screens. Also, for a given size, it is more marked as the shadow mask's radius of curvature is larger, i.e., as the tube is flatter, which is considered preferable for visual perception.